Structure of Membranes - Membrane Biophysics, Biological and Medical Physics, Biomedical Engineering - page 9

Chapter 2
Structure of Membranes
2.1 The Membrane as a Barrier and a Transporter
Amphipathic molecules adsorb themselves onto air-water or oil-water interfaces,
such that their head groups are facing the water environment. They aggregate to
form either spherical micelles or liquid crystalline structures. In general, amphipathic
molecules can be anionic, cationic, non-ionic, or zwitterionic. The relative con-
centrations of these surfactants in an aqueous solution will affect the solution's
physical and chemical properties. At a specific value, called the critical micelle
concentration, micelles containing 20-100 molecules are formed spontaneously in
the solution, with the hydrophilic head groups exposed and the hydrophobic tails hid-
den inside the micelle. The principal driving force for micelle formation is entropic,
due to a negative free energy change accompanying the liberation of water molecules
from clathrates. When phospholipids are mixed in water, they form double-layered
structures, since their hydrophilic ends are in contact with water while the hydropho-
bic ends face inwards touching each other.
Membranes have unique amphipathic properties since they possess both hydropho-
bic and hydrophilic parts. As described in Chap. 1 , the cell membrane is the thin,
nearly invisible structure that surrounds the cytoplasm of the cell. It is a continu-
ous boundary region that completely surrounds the cell, and which also connects
the endoplasmic reticulum and the nuclear membrane. Membranes are composed of
phospholipids, glycolipids, sterols, fatty acid salts, and proteins. The tails that come
off of the sphere represent the hydrophobic (or water-fearing) end of the phospho-
lipid. The two long chains coming off the bottom of this molecule are made up of
carbon and hydrogen. Because both of these elements share their electrons evenly,
these chains have no net electrostatic charge. Non-polar molecules are not attracted
to water; as a result water molecules tend to push them out of the way as they are
attracted to each other. This causes molecules with no electrostatic charge not to
dissolve in water. At the other end of the phospholipid there is a phosphate group
and several double-bonded oxygens. The atoms at this end of the molecule are not